The sweater in the picture IMHO is beautiful in reverse stockinette. You asked why not garter stitch? Well, garter has some draw backs. I love garter and will go to the trouble of using it, but it is slower than using the reverse stockinette, and it uses more yarn I think. Garter of course has the little lines going across it, or ridges as they are called, whereas the reverse stockinette is more smooth (or evenly bumpy may describe it better.
). My knitting guru told me once that she thought garter stitch was one of the hardest to do well. I've discovered what she means, any variation in your gauge will show up in garter more than in reverse stockinette. Garter is also more stretchy, and of course reverse stockinette is just as stable as stockinette, because it is, only turned over.
But it is a design choice. It is your sweater, you can do whatever you like the best. If you want to do garter instead of reverse stockinette you may need more yarn. Or stockinette is fine too. But for my money, for this design the stitch they chose is part of what makes it so attractive. Too bad a person can't do it so it is reversible so you could have your cake and eat it too.
I like that idea.